Plating adherent metal coatings onto polymethyl methacrylate materials

ABSTRACT

A simple and inexpensive method of producing adherent plated coatings on transparent acrylics such as polymethyl methacrylate. The polymethyl methacrylate surface is initially thoroughly abraded with a very fine abrasive and then immersed in methanol. A copper layer is then electrolessly deposited, and one or more metal layers electrodeposited on top of the copper. In a preferred embodiment, successive layers of copper, nickel and chromium are electrodeposited onto the copper layer. Plated designs on a transparent background are produced by etching away unwanted portions of the metal layers, and buffing the exposed abraded plastic surface, to restore the surface to its original luster and transparency. The plated design is adherent enough to withstand the buffing.

United States Patent 11 1 Wheatley Oct. 28, 1975 [5 PLATING ADHERENT METAL COATINGS 3,758,388 9/1973 Shotton 117/47 A ()NTO POLYMETHYL METHACRYLATE 3,791,939 2/1974 Ferrara et al. 117/212 X MATERIALS Marion E. Wheatley, Warren, Mich.

General Motors Corporation, Detroit, Mich.

Filed: May 24, 1974 Appl. No.: 473,272

Inventor:

Assignee:

US. Cl. 204/15; 427/264; 427/290; 427/265; 427/306; 427/404; 427/367; 204/30 Int. Cl.'.... C23C 3/02; C25D 5/00; C25D 5/14 Field of Search 1 17/47 A, 212, 64 R; 204/30, 32 R, 15

References Cited UNITED STATES PATENTS Primary ExaminerRalph S. Kendall Assistant Examiner-John D. Smith Attorney, Agent, or Firm-Robert .1. Wallace [57] ABSTRACT A simple and inexpensive method of producing adherent plated coatings on transparent acrylics such as polymethyl methacrylate. The polymethyl methacrylate surface is initially thoroughly abraded with a very fine abrasive and then immersed in methanol. A copper layer is then electrolessly deposited, and one or more metal layers electrodeposited on top of the copper. In a preferred embodiment, successive layers of copper, nickel and chromium are electrodeposited onto the copper layer. Plated designs on a transparent background are produced by etching away unwanted portions of the metal layers, and buffing the exposed abraded plastic surface, to restore the surface to its original luster and transparency. The plated design is adherent enough to withstand the buffing.

3 Claims, No Drawings PLATING ADHERENT METAL COATINGS ONTO POLYMETHYL METHACRYLATE MATERIALS BACKGROUND OF THE INVENTION This invention relates to metal plating plastics, and more particularly to an improved method of metal plating transparent acrylic plastic materials such as polymethyl methacrylate.

A wide variety of both opaque and transparent plastic materials have become commercially available. For many applications it is desirable to metal plate these plastics. Considerable progress has been made with most of these plastics in producing adherent metal deposits on them. Adherent metal plating has been made possible through a variety of substrate pretreatments, including single and multiple etches, and dips in various conditioners and solvents. Moreover, many of these plastics have become available in special formulations which are either directly plateable or minimize the extent of pretreatment required for metal plating.

In most of these developments, adherent metal plating is obtained at the expense of surface finish in the underlying plastic substrate. This is not a significant problem where blanket coatings are desired, or the plastic material is opaque.

On the other hand, most clear plastic materials, particularly acrylics such as polymethyl methacrylate, are used for their transparent characteristics. Lucite, Acrylate, and plexiglas are examples of commercially available polymethyl methacrylate plastics. If they are to be plated, they are generally selectively plated to form a pattern of some sort on their surface, as for example letters, numerals, figures, surface contour enhancement, and the like. Clarity and transparency of the unplated surface must not be deleteriously affected in the plating process. Polymethyl methacrylate materials can be readily selectively plated under commercial production conditions by vapor deposition through a mask. However, such coatings are poorly adherent. More adherent coatings can be produced on polymethyl methacrylate materials by initially applying a plateable lacquer through a mask. The plateable lacquer presents a plateable surface which can then be subsequently electroplated in the normal and accepted manner. However, this latter technique is expensive. Moreover, it is limited in use. Each discrete segment in a pattern must be individually connected to a source of electrical potential if it is to be electroplated.

Accordingly, considerable progress has been made in the metal plating of opaque plastics. However, the selective metal plating of transparent plastics such as polymethyl methacrylate is another matter.

I have found a simple, reliable and inexpensive technique for producing very adherent deposits on acrylics such as polymethyl methacrylate. Moreover, my technique can be used to selectively metal plate acrylics such as polymethyl methacrylate without deleteriously aflecting the acrylic substrate surface finish.

OBJECTS AND SUB/ MARY OF THE INVENTION An object of this invention is to provide an improved method for adherently metal plating transparent acrylic plastics such as polymethyl methacrylate.

A further object is to provide an improved method for selectively adherently metal plating transparent acrylic plastics such as polymethyl methacrylate.

The objects of this invention are obtained by initially abrading the polymethyl methacrylate surfaces with an abrasive of about 600 grit size, dipping the abraded surface in methanol, and then the electrolessly plating the treated surface with copper. At least one additional metal layer is electrodeposited onto the copper in the normal and accepted manner. To form a metal pattern on the polymethyl methacrylate surface, the metal coating is selectively photoetched away. This exposes the underlying abraded polymethyl methacrylate surface. The entire surface including the metal deposit is then buffed to restore the polymethyl methacrylate surface to its original luster and transparency.

Other objects, features and advantages of this invention will become more apparent from the following description of preferred examples thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS:

The surface of a sheet of a commercially available polymethyl methacrylate plastic such as Acrylite is used as a substrate. It is thoroughly sanded with a 600 grit emery paper. It is first sanded with a plurality of longitudinally mutually parallel strokes and then with a plurality of longitudinal mutually parallel strokes perpendicular the first. Afterwards, the substrate surface is washed with water to remove the dust from sanding. The sanded and rinsed polymethyl methacrylate substrate is dipped in methanol for about 10 seconds. Spray rinsing with methanol can be used instead of dipping the substrate in a methanol bath. The polymethyl methacrylate substrate is then rinsed with water. Following the rinse, it is sensitized, activated, and electrolessly plated with copper in the normal and accepted manner. For example, the substrate can be dipped in an aqueous solution containing 159 grams per liter stannous chloride for about 4 minutes, and then rinsed with water. It is then immersed in an aqueous solution containing 7.5 grams per liter palladium chloride for about 4 minutes, and again rinsed with water. The polymethyl methacrylate substance is then immersed for about 10 minutes in an aqueous electroless copper solution of the following composition:

CuSO, SI'I O Na CJ-LO 2H O 15 grams per liter 30 grams per liter NaOH 14 grams per liter P" 13.2 Fon'naldehyde 10 ml per liter CuSO. 511,0 225 grams per liter II,SO. 56 grams per liter Cu (as metal) 58 grams per liter Cl (as HCl) 20 parts per million Brighteners 3.8 grams per liter A 0.025 mm thick layer of copper is electrodeposited at a current density of 4.3 amperes per square decime- NiSO. 6H O 338 grams per liter NiCl 3H O 60 grams per liter H 80 47 grams per liter NaBF 1.2 grams per liter Brighteners 3% by volume After nickel plating the substrate is rinsed with water and immersed in a chromium plating bath of the following composition:

248 grams per liter 2.48 grams per liter CrO H SO.

A flash blanket coating of chromium, less than about 0.000025 mm thick can be used. It is deposited at a current density of about 23 amperes per square decimeter for about 3 minutes at a temperature of about 41 C.

A photosensitive coating, a resist, is then applied to the chromium plated surface by brushing, spraying or the like. One photosensitive coating which can be used is Dynachem 3140. The photosensitive coating is selectively exposed through a pattern mask and the resultant pattern developed. This leaves an etch resist mask selectively on the metal surface of the plated substrate, corresponding to the metal pattern desired. The sub strate is then immersed for about 2 minutes in a room temperature aqueous solution containing 50 volume percent concentrated hydrochloric acid. This selectively etches away the unwanted portions of the chromiurn layer. The masked substrate is then rinsed, and spray etched for about 4 minutes at room temperature with a 42 Baurne ferric chloride solution. This removes the unwanted portions of the copper and nickel layers. The masked substrate is then rinsed again and dried. After drying, the resist is removed with an organic solvent, and the substrate dried again. An adherent metal pattern is now defined and exposed on an abraded background of polymethyl methacrylate.

The entire surface of the substrate, including the metal pattern, is then buffed to remove the sanding marks, and restore the polymethyl methacrylate surface to its original luster and transparency. Buffing can be in one direction or in two orthogonal directions without any buffing compound, using an 8 inch diameter, 3 inch wide looseleaf cloth buffing wheel rotating at about 1800 rpm.

The result is a highly reflective adherent metal pattern on a highly transparent polymethyl methacrylate background with a surface finish rivaling the original unplated material.

As is evident the adherence of the metal deposit to the polymethyl methacrylate surface is so strong that it can withstand the rigors of bufi'mg that are used to restore the substrate back to its original luster and transparency.

The grit size used for the initial mechanical abrasion is important. It should be fine enough to permit surface restoration by buffing, and yet not so fine that it will not produce good metal adhesion. I have found that a 400 grit size produces good adhesion but the optical properties of the substrate are adversely affected. On the other hand, with a grit size of about 800 or smaller, good adhesion of the metal plate is not obtained.

In the preceding example of the invention, the polymethyl methacrylate surface was thoroughly sanded in orthogonal directions with a fine emery paper. The sanding direction is not as important as thorough coverage, and it can be conducted as either a wet or dry treatment.

In the preceding example, it was mentioned that methanol was used as a surface conditioner after the surface abrasion. However, no other low molecular weight alcohol or glycol was found to produce results equal to pretreatment with methanol.

The polymethyl methacrylate substrate need not be soaked in methanol any appreciable period of time before it is rinsed off. The substrate can be dipped in methanol or the methanol may be sprayed on. After immersion or spraying, it can be rinsed off inunediately, e.g., within two or three seconds. Generally I prefer to rinse the methanol ofi' after 5-10 seconds to insure consistent results. Longer waiting times appear to provide no added benefit. Also, the substrate can be plated without rinsing off the methanol. However, this is not preferred. In this latter connection, rinsing after all of the steps of this invention is no more important to this invention than it is to any other metal finishing process.

The particular manner of buffing used is not especially critical. The surface can be buffed first in one direction and then orthogonally in another direction. On the other hand, severe buffing is generally not preferred. By severe buffing I means bufiing which produces extensive removal of the polymethyl methacrylate surface. This can deleteriously affect its optical properties. If the surface is initially abraded with a grit such as previously described, severe buffing is unnecessary. Abrasive marks can easily be removed and the original surface finish restored without deleteriously affecting original optical properties of the polymethyl methacrylate substrate.

The electroless copper layer provides a basis upon which subsequent electrodeposits can be made. It need only be thick enough to insure a continous coating.

It should also be noted that an electrodeposited metal overlay is always used in forming decorative patterns on polymethyl methacrylate materials in accordance with this invention. The electroplated coating is used to not only protect the electroless deposited layer from the rigors of buffing but also can be used to provide added corrosion resistance and difi'erent decorative effects. It should be thick enough so that the buffing will not cut through it, and not so thick that it is difficult to satisfactorily buff the exposed contiguous portions of the polymethyl methacrylate surface. Electroplated coating thicknesses of about 0.038 mm to 0.13 m can be used. Thicker deposits than described herein are generally not desired because they increase costs and usually add no significant functional benefit.

I claim:

1. A method of enhancing adherence of plated metal coatings on polymethyl methacrylate substrates comprising the steps of:

abrading an entire polymethyl methacrylate surface area where enhanced metal adherence is desired with an abrasive of about 600 grit size,

applying methanol to said abraded surface area,

electrolessly plating a layer of copper onto said surface area, and

electroplating a layer of a metal that adheres to copper onto said copper layer.

2. A method of producing a more adherent plated pattern on the surface of a polymethyl methacrylate transparent body without deleteriously affecting optical properties of said surface comprising the steps of:

abrading a surface of a polymethyl methacrylate body with an abrasive of about 600 grit size, applying methanol to said surface,

electrolessly depositing a blanket layer of copper onto said surface,

electroplating an additional blanket layer of a metal that adheres to copper onto said electrolessly deposited copper layer,

selectively etching away congruent portions of said layers to define an adherent metal pattern on said surface and concurrently expose selected parts of the abraded substrate surface area, and buffing said polymethyl methacrylate body surface with the metal pattern thereon to remove abrasive marks in said exposed surface parts and substantially restore original surface finish to these parts without deleteriously affecting the metal pattern. 3. A method of producing a bright, adherent, and

wear resistant plated pattern on the surface of a polymethyl methacrylate transparent body without deleteriously affecting optical properties of said surface comprising the steps of:

completely abrading an entire preselected area on a surface of a polymethyl methacrylate body, using an abrasive of about 600 grit size, immersing said body in methanol, electrolessly depositing a blanket layer of copper onto said surface, electroplating a blanket layer of copper onto said electrolessly deposited copper layer, electroplating a blanket layer of nickel onto said electroplated copper layer, electroplating a blanket layer of chromium onto said electroplated nickel layer, selectively etching away congruent portions of all said layers to define a bright, adherent, and wear resistant metal pattern on said surface and concurrently expose selected pans of the abraded substrate surface area, and buffing said polymethyl methacrylate body surface with the metal pattern thereon to remove abrasive marks in said exposed surface parts and substantially restore original surface finish to these parts without deleteriously affecting the metal pattern. 

1. A method of enhancing adherence of plated metal coatings on polymethyl methacrylate substrates comprising the steps of: abrading an entire polymethyl methacrylate surface area where enhanced metal adherence is desired with an abrasive of about 600 grit size, applying methanol to said abraded surface area, electrolessly plating a layer of copper onto said surface area, and electroplating a layer of a metal that adheres to copper onto said copper layer.
 2. A method of producing a more adherent plated pattern on the surface of a polymethyl methacrylate transparent body without deleteriously affecting optical properties of said surface comprising the steps of: abrading a surface of a polymethyl methacrylate body with an abrasive of about 600 grit size, applying methanol to said surface, electrolessly depositing a blanket layer of copper onto said surface, electroplating an additional blanket layer of a metal that adheres to copper onto said electrolessly deposited copper layer, selectively etching away congruent portions of said layers to define an adherent metal pattern on said surface and concurrently expose selected parts of the abraded substrate surface area, and buffing said polymethyl methacrylate body surface with the metal pattern thereon to remove abrasive marks in said exposed surface parts and substantially restore original surface finish To these parts without deleteriously affecting the metal pattern.
 3. A METHOD OF PRODUCING A BRIGHT, ADHERENT, AND WEAR RESISTANT PLANTED PATTERN ON THE SURFACE OF A POLYMETHYL METHACRYLATE TRANSPARENT BODY WITHOUT DELERIOUSLY AFFECTING OPTICAL PROPERTIES OF SAID SURFACE COMPRISING THE STEPS OF: COMPLETELY ABRADING AN ENTIRE PRESELECTED AREA ON AASURFACE OF A POLYMETHYL METACRYLATE BODY, USING AN ABRASIVE OF ABOUT 600 GRIT SIZE, IMMERSING SAID BODY IN METHANOL, ELECTROLESSLY DEPOSITING A BLANKET LAYER OF COPPER ONTO SAID SURFACE, ELECTROPLATING A BLANKET LAYER OF COPPER ONTO SAID ELECTROLESSLY DEPOSITED COPPER LAYER, ELECTROPLATING A BLANKET LAYER OF NICKEL ONTO SAID ELECTROPLATED COPPER LAYER, ELECTROPLATING A BLANKET LAYER OF CHROMIUM ONTO SAID ELECTROPLATED NICKEL LAYER, SELECTIVELY ETCHING AWAY CONGRUENT PORTIONS OF ALL SAID LAYERS TO DEFINE A BRIGHT, ADHERENT, AND WEAR RESISTANT METAL PATTERN ON SAID SURFACE AND CONCURRENTLY EXPOSED SELECTED PARTS OF THE ABRADED SUBSTRATE SURFACE AREA, AND BUFFING SAID POLYMETHYL METACRYLATE BODY SURFACE WITH THE METAL PATTERN THEREON TO REMOVE ABRASIVE MARKS IN SAID EXPOSED SURFACE PARTS AND SUBSTANTIALLY RSTORE ORIGINAL SURFACE FINISH TO THESE PARTS WITHOUT DELETERIOUSLY AFFECTING THE METAL PATTERN. 